This invention relates to the field of surveillance systems, and in particular to a video content analysis system that includes cameras situated to provide a downward-looking view, such as cameras used for counting persons entering and/or leaving facilities.
Video surveillance systems are commonly used to monitor access to facilities. In some applications, video cameras are deployed to count the number of people entering and leaving an area. In such applications, the camera is often mounted in a ceiling adjacent a doorway through which people must pass to enter or leave the area, and oriented with a vertical field of view. A video processing system analyzes the images from this downward-viewing camera to distinguish and count individuals.
To provide an accurate count, the individuals need to be tracked as they pass beneath the camera, to determine whether they are entering or leaving, and to detect ‘false counts’, wherein individuals pass beneath the camera in one direction, then back up or turn around, and do not complete the transit through the doorway. Because the camera is oriented vertically, however, and because the distance between the camera an the individual's head is relatively small, the image of the individual substantially changes as the individual enters the camera's field of view, passes beneath the camera, then exits the camera's field of view.
Because of the substantial changes in the image shape and size as an individual approaches and passes beneath an overhead camera, and the need to track each individual to maintain an accurate count, most image processing systems designed for such counting require a dynamic scaling of objects in an image, based on the camera's angle of view to the object.
FIG. 1 illustrates an example configuration of an overhead camera 110 mounted a distance d above a floor plane 120, with a field of view that is substantially orthogonal to the floor plane. Two positions 150, 150′ of a person are illustrated. At position 150, the person is in line with the centerline of the camera's field of view, and using the floor surface 120 as a reference plane, will appear in the camera image as wide as an object of width W on the floor. At position 150′, the person is offset by an amount p from the centerline of the camera, and will appear in the camera image as wide as an object of width W′ on the floor. On the camera image plane, which is parallel to the floor reference plane, the ratio of pixel line lengths corresponding to the images of the person at locations 150, 150′ will be equal to the ratio of widths W′/W.
The increase in width W′ as a function of the distance p from the camera centerline is dependent primarily on two factors: the height, h, of the person, and the “optical” height, d, of the camera. The term “optical height” takes into account the optical characteristics of the camera and its lens. That is, in the example of FIG. 1, the illustrated field-of-view lines that define the projection of the person on the floor are shown as converging to a point 101. In reality, the point at which these lines effectively converge, the effective focal point, is determined by the camera's optical characteristics.
It is an object of this invention to provide a method and system for determining the effective optical height of a camera. It is a further object of this invention to provide a method and system for determining a scaling function for use in a tracking system that uses a vertically oriented camera.
These objects, and others, are provided by a calibration method and system that facilitates determination of an effective focal point of a vertically oriented camera. Two equal-length reference lines at different heights relative to a reference plane are imaged. By comparing the resultant image-width of each of these lines in the view provided by the camera, the effective focal point of the camera is determined. In a preferred embodiment, a doorframe is used to provide equal-length lines at different heights parallel to a floor plane. The threshold of the doorway at the floor plane is used as a reference line, and a parallel calibration line at a known/measured height in the doorway is defined. The image produced by a vertically oriented camera of these two equal-length lines will show a longer line at the elevated position, the different widths of the images of these lines being dependent upon the camera's effective focal point. From these two projections of the equal-length lines at different heights, the camera's focal point distance from the reference floor plane is determined.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.